The present invention relates to a fuel assembly for a boiling water reactor (BWR). In particular, the invention is concerned with a fuel assembly suitable for being loaded into a reactor core wherein a water gap width on a side (control rod side) where a control rod is inserted and a water gap width on a side (opposite to the control rod side) where a control rod is not inserted, are almost equal to each other. This type of core is usually called a xe2x80x9cC-lattice corexe2x80x9d.
Recently, in a light water reactor (LWR), improvement in fuel economy and reduction of spent fuel (reduction in the number of fuel assemblies discharged from the reactor and reduction in the amount of spent fuel) have received considerable attention. To this end, it is effective to enhance discharge burn-up (high burnup) of fuel assemblies.
For the high burnup, it is necessary to increase uranium enrichment which means uranium-235 enrichment. The increase in uranium enrichment causes an increase in the hot-to-cold reactivity swing and a decrease in the reactor shut-down margin. The hot-to-cold reactivity swing is a reactivity difference of the reactor between a hot operating condition and a cold shut-down condition.
In addition, the increase in uranium enrichment causes an increase in the exposure (burnup) difference between fuel assemblies due to a residence (loaded) period difference in a reactor core. This increase in the exposure difference causes an increase in the maximum thermal power (or channel peaking factor) of fuel assemblies and a decrease in thermal margin. The thermal margin is a difference between an operating limit of thermal power and an actual maximum thermal power in an operating condition.
Japanese Laid-open No. Hei 8-285977 describes a short-length (part-length) fuel rod containing a burnable absorber, which absorbs thermal neutrons causing the nuclear reaction and is disposed at a corner position in the second layer from the outside of a fuel assembly to thereby enhance the thermal margin while attaining high burnup.
Japanese Laid-open No. Sho 64-31091 describes a fuel assembly for being loaded into a D-lattice core wherein a water gap width on a control rod side is larger than that on an opposite side, wherein an inner water gap is formed to increase the reactor shut-down margin, and the inner water gap or a water rod is shifted toward the side opposite to the control rod side to ensure the thermal margin.
However, in JP 8-285977, no consideration is given to improving of the reactor shut-down margin, and in JP 64-31091, a fuel assembly is disclosed that has a D-lattice core. Accordingly, no consideration is given to a fuel assembly for a reactor core wherein the water gap width on the control rod side and that on the opposite side are almost equal to each other.
It is a first object of the present invention to provide a fuel assembly which, when loaded into a reactor core wherein the control rod-side water gap width and the opposite-side water gap width are almost equal to each other, can attain high burnup and an increase in the reactor shut-down margin.
It is a second object of the present invention to provide a fuel assembly which can achieve the above first object and can also increase the thermal margin.
In accordance with the present invention, a fuel assembly is suitable to be loaded into a reactor core that has a control rod-side water gap width and an opposite-side water gap width that are almost equal to each other.
The fuel assembly has a plurality of fuel rods arranged in a square lattice pattern, each fuel rod being filled with nuclear fuel pellets and also has at least one neutron moderator rod shifted toward one corner where a control rod is inserted away from a cross sectional center of the fuel assembly. That is, a center of the at least one neutron moderator rod is shifted toward one corner where a control rod is inserted with respect to the cross sectional center of the fuel assembly.
In accordance with the present invention, a fuel assembly has a fuel bundle having a plurality of fuel rods arranged in a square lattice pattern and at least one neutron moderator rod, each fuel rod being filled with nuclear fuel pellets. An upper tie plate and a lower tie plate hold upper end portions and lower end portions of the fuel rods respectively. A channel box covering the fuel bundle is fixed to one corner of the upper tie plate by a channel fastener. A center of the at least one neutron moderator rod is shifted toward the one corner from a cross sectional center of the fuel assembly.
Preferably, the plurality of fuel rods include a plurality of short-length (part-length) fuel rods having a shorter active fuel length than the remaining fuel rods and the number of the short-length fuel rods are arranged in one diagonally divided half area opposite to the one corner that is larger than that in the other diagonally divided half area.
Further, preferably, at least one of the short-length fuel rods is disposed at a position adjacent to the at least one neutron moderator rod on a side opposite to the one corner.
Preferably also, the fuel assembly is divided into a first region including the one corner and a second region by a diagonal line in a cross section, and an average uranium enrichment of the fuel rods in the second region is higher than that of the fuel rods in the first region.
Still further, preferably, the plurality of fuel rods include a plurality of burnable absorber-filled fuel rods added burnable absorber thereto and the number of said burnable absorber-filled fuel rods being arranged in one diagonally divided half area opposite to said one corner is larger than that in the other diagonally divided half area.
In accordance with a preferred embodiment of the present invention, a fuel assembly has a plurality of fuel rods arranged in a square lattice of 10-rows by 10-columns for each fuel rod being filled with nuclear fuel pellets and also has one water rod disposed in an area of 3-rows by 3-columns in the square lattice. A center of the one water rod is shifted toward one corner where a control rod is inserted with respect to a cross sectional center of the fuel assembly.
Also, in accordance with a preferred embodiment of the present invention, a fuel assembly has a fuel bundle having a plurality of fuel rods arranged in a square lattice of 10-rows by 10-columns and one water rod disposed in an area of 3-rows by 3-columns in said square lattice, each fuel rod being filled with nuclear fuel pellets and an upper tie plate and a lower tie plate holding upper end portions and lower end portions of the fuel rods respectively. A channel box covering the fuel bundle is fixed to one corner of the upper tie plate by a channel fastener. A center of the one water rod is shifted toward the one corner with respect to a cross sectional center of the fuel assembly.
As a result of the present invention, since the at least one neutron moderator rod (or one water rod) is shifted toward one corner (the control rod side and the channel fastener side), where the control rod is inserted and the channel fastener is fixed, from a cross sectional center of the fuel assembly, thermal neutron flux of one diagonally divided half area opposite to the one corner becomes higher than that of the other diagonally divided half area.
Therefore, the control rod worth, which means the ability of the control rod to control the nuclear reaction depending on the thermal neutron flux, can be enhanced in comparison with the case where a center of the neutron moderator rod is disposed at a center of the fuel assembly or shifted toward the other corner opposite to the one corner. The enhancement of the control rod worth contributes to an increase in the reactor shut-down margin while attaining the high burnup.
Furthermore, since the short-length fuel rods are disposed in a larger number on a side opposite to the channel fastener side than on the channel fastener side, moderator (water) distribution on both sides becomes almost uniform over the cross section of the fuel assembly in an upper region above the upper end of the short-length fuel rods when the reactor is in a hot operating condition. This moderator distribution contributes to the flattening of local power distribution.
In addition, it is possible to decrease a rise in reactivity because neutron flux is over-moderated based on a great increase in water density on the opposite side in the upper region above the upper end of the short-length fuel rods when the reactor is in a cold shut-down condition. This decrease in the reactivity contributes to an increase in the thermal margin.
Furthermore, since at least one of the short-length fuel rods is disposed at a position adjacent to the at least one neutron moderator rod on the side opposite to the one corner, it is possible to decrease an absolute value of the void coefficient.
Furthermore, since the average uranium enrichment in the first region where the thermal neutron flux becomes relatively high under the influence of the at least one neutron moderator rod is set low and the average uranium enrichment in the second region where the thermal neutron flux becomes relatively low is set high, the moderator distribution can be made flatter in the hot operating condition and it is possible to further increase the thermal margin.
Furthermore, since the thermal neutron flux on the channel fastener side increases relatively under the influence of the burnable absorber-filled fuel rods arranged in a larger number on a side opposite to the channel fastener side, it is possible to further enhance the control rod worth and further increase the reactor shut-down margin.
In addition, since the burnable absorber-filled fuel rods are disposed in a larger number on the opposite side where the thermal neutron flux is relatively low, the effect of the burnable absorber is maintained for a long period. This long period effect is suitable for a long-term cycle operation of the reactor.